Troubleshooting Left Side Running Light Problem on a Boat Trailer after Installing a New Tail Light

Updated 10/18/2010 | Published 10/17/2010

Products Featured in This Question

Combination LED Trailer Tail Light - Submersible - 7 Function - 14 Diodes - Driver Side

(1285)

$23.50

Hook Probe Circuit Tester

(66)

$14.36

Question:

On my boat trailer, the left side running light will not work, The left side brake light and turn signal work fine. All 3 functions work correctly on the right side. I installed a new Optronics low profile light on the left side and I still have the same problem. There is a side light at the front of the trailer that works, however there is a second side light located at the fender well that does not work. I can see nothing obvious wrong with the wiring, but most of the wiring is inside the trailer itself. Any suggestions? Thanks.

asked by: Al

Expert Reply:

It sounds as if you either have a weak ground at the first light on the left side of the trailer that is not working or you have a broken or pinched wire on the left side after the light that is working. You have effectively eliminated the vehicle and trailer connectors by observing the fact that the first running light inline on the left side of the trailer is functioning properly and that all of the other trailer lighting is working.

Start, by making sure the lights that are not working are securely grounded to a clean, rust, corrosion and paint free part of the trailer frame. Then check the brown wire to see where your 12-volt power is being lost, with a circuit tester, item # PTW2993 if needed.

If there is not power past the splice at the first running light, inside the frame, the wire may be disconnected, so power is not feeding on through to the more rearward lights. I would try pulling the running light that is working and checking the connection that is there. If this is not the problem I would then run a new piece of wire, item # 12-1-1, through the trailer frame from the first light to the second and splice it in where the original wire was spliced in at both lights. Use the original wire to pull the new wire through the frame. this should provide power to the second light. At this time make sure the first and second light are grounded to a clean, rust, corrosion and paint free part of the trailer frame.

If the fender light is now working and the tail light is still not functioning, run a new wire in the same manner, through the trailer frame to the tail light, and make sure the tail light is properly grounded. You should then have working tail lights on the left side of the trailer.

We have typical trailer wiring diagrams on our Wiring FAQ page, see link, for you to review.

expert reply by: Robert G